The invention relates to an acoustic mirror that reflects acoustic waves within a defined frequency range.
An acoustic mirror of this type is described, for example, in the reference by B. Olutade et al., titled xe2x80x9cSensitivity Analysis of Thin Film Bulk Acoustic Resonator Ladder Filterxe2x80x9d, IEEE International Frequency Control Symposium 1997, 737. The acoustic mirror contains a stack of thin films with alternately high and low acoustic impedances. The thickness of the thin films is, for example, a quarter of the wavelength of the acoustic waves that are to be reflected in the respective thin film. It is preferable for the production of the acoustic mirror to be as compatible as possible with silicon process techniques, so that the acoustic mirror can be integrated with other components on a chip.
It is known from the reference by Y. Hayashi et al., titled xe2x80x9cNitride Masked Polishing (NMP) Technique for Surface Planarization of Interlayer-Dielectric Filmsxe2x80x9d, Ext. Abstr. of the 1992 Int. Conf. on Solid State Devices and Materials, Tsukuba (1992), 533, that in order to produce a surface which is as planar as possible for a layer of borophospho-silicate glass (BPSG), which covers an elevated cell array and a periphery of a memory cell configuration, to apply a stop layer of silicone nitride to a part of the BPSG layer above the periphery and then to planarize the layer by chemical mechanical methods. Prior to the planarization, part of the BPSG layer above the cell array is elevated. On account of the stop layer, the part is removed selectively with respect to the part of the BPSG layer that lies above the periphery during planarization.
It is accordingly an object of the invention to provide an acoustic mirror and a method for producing the acoustic mirror which overcome the above-mentioned disadvantages of the prior art devices and methods of this general type, whose production is compatible with silicon process techniques and which can have a particularly high reflectivity compared to the prior art.
With the foregoing and other objects in view there is provided, in accordance with the invention, an acoustic mirror. The acoustic mirror contains a substrate, and a layered sequence formed of at least two insulating layers and at least two metal layers disposed alternately on top of one another on the substrate. The insulating layers have substantially equal thicknesses and the metal layers have substantially equal thicknesses. The thicknesses of the insulating layers and of the metal layers are such that a reflection condition is satisfied. An auxiliary layer having at least one recess formed therein, is provided, and at least the metal layers are disposed inside the at least one recess.
The greater the difference between the acoustic impedances of the layers of the acoustic mirror, the higher the reflectivity becomes. Metal layers have a particularly high impedance. The reflectivity of the acoustic mirror can be particularly high on account of the use of metal layers.
Acoustic waves are reflected by a stack that contains the insulating layers and the metal layers. The reflection condition which is satisfied by the thicknesses of the layers consists, for example, in the fact that an acoustic wave which has a defined frequency and impinges on the substrate at a defined angle is reflected particularly well by the layers.
The thicknesses of the insulating layers and of the metal layers are preferably in each case substantially xcex/4, where xcex is the wavelength of an acoustic wave in the respective layer which is reflected particularly well if it impinges on the substrate at right angles. Given the same frequency of the wave, the wavelength is dependent on the impedance and on the specific density of the material passing the wave.
The insulating layers have a low acoustic impedance, while the metal layers have a high acoustic impedance. The thicknesses of the metal layers are therefore preferably identical. The same applies to the insulating layers. However, the insulating layers and the metal layers are generally of different thicknesses.
The lowermost layer in the stack is, for example, one of the insulating layers.
The uppermost layer in the stack is preferably one of the metal layers.
The layers of the stack are applied substantially conformally.
Adjacent layers of the stack preferably bear against one another. However, it is also possible for interlayers, for example diffusion barriers, which are very thin in relation to the other layers, to be disposed between the adjacent layers.
Since the metal layers are produced in at least one recess, the metal layers have smaller horizontal areas than the substrate. The capacitance of the acoustic mirror, which is formed by the metal layers, is smaller than an acoustic mirror in which the metal layers cover the entire substrate.
A metal layer that covers the entire substrate, on account of a high layer stress, causes bending of the substrate, which increases as the thickness of the metal layer rises. In the acoustic mirror according to the invention, bending of the substrate is avoided, since the metal layers do not cover the entire substrate, but rather are only disposed in one or more recesses.
The metal layers are in each case initially deposited over the entire surface and then structured by chemical mechanical polishing in such a way that they are disposed inside the respectively associated recess. The metal can be structured more effectively by chemical mechanical polishing than by etching with the aid of a photolithography process. The reason for this is the bending of the substrate after deposition of the metal layer over the entire surface, which leads to problems with photolithographic processes. By way of example, in the event of substantial bending it is impossible to find alignment marks in order to align masks. Furthermore, in the event of bending of the substrate, sharp focusing during exposure to produce the masks from photoresist is no longer possible. In the method according to the invention, the recess can be produced with the aid of a photolithographic method prior to deposition of the metal layer, i.e. with an unbent substrate. The shape of the recess defines the shape of the metal layer.
The method allows the simultaneous production of contacts or lines, which are likewise produced on the substrate, by corresponding structuring of the auxiliary layer and by filling the recesses or contact holes produced by the deposition and the chemical mechanical polishing of the metal layer. In this case too, the required lithography takes place with an unbent substrate.
As a result of the provision of an auxiliary layer which surrounds the metal layer, the difference in height between a region in which the metal layer is disposed and a region that is disposed next to the metal layer is nonexistent or lower than if the metal layer had been structured by etching. In a method according to the invention, therefore, no topology problems are encountered. In contrast, when metal is etched with the aid of a photolithography method to produce the metal layer without the use of the auxiliary layer and of the recess, a step is formed, which causes problems during the deposition of further layers or during photolithographic methods, in which both a region of the structured metal layer and an adjoining region are exposed. Considerable differences in height cause problems in particular for photolithography process steps, since even when the photoresist is being applied by centrifuging it is impossible to produce a homogeneous layer without fluctuations in thickness. Photolithographic steps of this type are required, for example, if electrodes, for example, are to be applied to the acoustic mirror.
One of the metal layers may be disposed in a recess in which there is otherwise no further metal layer. In this case, after the recess has been produced, the metal layer is deposited and is planarized by chemical mechanical methods. If a further metal layer is to be produced above the metal layer, one of the insulating layers is produced on the auxiliary layer, followed by a further auxiliary layer with a recess which is disposed above the recess in the auxiliary layer and passes all the way through the further auxiliary. layer. The further metal layer is produced in the recess in the further auxiliary layer.
No further metal layer and none of the insulating layers are disposed inside the recess. Since the metal layer is structured by chemical mechanical polishing, the depth of the recess is greater than or equal to the thickness of the structured metal layer. It is preferable for the metal layer to substantially fill the recess.
However, an upper surface of the metal layer may also lie at a lower level than an upper surface of the auxiliary layer. In this case, the metal layer and a part of the insulating layer disposed above it substantially fill the recess. Since the insulating layer is disposed on the auxiliary layer and the metal layer, in this case too the insulating layer is not disposed inside the recess. Only part of the insulating layer is disposed inside the recess.
It is possible, for example, for all the metal layers to be produced in each case in a dedicated recess. The recesses are disposed on top of one another. One of the insulating layers is disposed between in each case two of the auxiliary layers.
To reduce the process outlay, it is advantageous if at least two of the metal layers and one of the insulating layers disposed there-between are produced in the same recess. For this purpose, after the recess has been produced, one metal layer is deposited over the entire surface, followed by the insulating layer and then the further metal layer. Then, the metal layers and the insulating layer are abraded by chemical mechanical polishing in one step until parts of the two metal layers and of the insulating layer which are disposed outside the recess have been removed.
The lowermost layer of the layers that are produced in the recess may be another of the insulating layers or one of the two metal layers.
To increase the reflectivity of the acoustic mirror, it is also possible for more than two metal layers to be produced in the same recess. For this purpose, the auxiliary layer has to be formed with an even greater thickness and further insulating layers and metal layers have to be deposited.
The depth of the recess is greater than or equal to the sum of the thicknesses of all the layers disposed inside the recess.
By way of example, a horizontal area of the metal layers is between (20*20) xcexcm2 and (400*400) xcexcm2. With areas of this size, a chemical mechanical polishing effect known as dishing may occur. In this event, during the thinning of large regions, central parts are abraded to a greater extent than outer parts. To prevent dishing, it is advantageous if a stop layer is applied to the metal layer that would be uncovered during the chemical mechanical polishing. The thickness of the stop layer is preferably between 30 nm and 100 nm or is at least five times smaller than the thickness of the metal layer. The stop layer is formed of a material that is scarcely attacked during the chemical mechanical polishing of the metal. Parts of the stop layer that are disposed outside the recess are removed. Then, the metal layer undergoes chemical mechanical polishing selectively with respect to the stop layer. In the process, the stop layer prevents dishing from also abrading parts of the metal layer that lie inside the recess. The depth of the recess should be not less than the sum of all the layers disposed therein, including the stop layer.
If a plurality of metal layers are produced in the same recess, the stop layer covers the uppermost metal layer.
The stop layer is formed, for example, of titanium, TiN or silicon nitride.
The stop layer can, for example, be structured by chemical mechanical polishing. Alternatively, the stop layer is structured by a photolithography method. A suitable method in this case is, for example, dry etching.
After structuring the metal layers, the stop layers disposed thereon can be removed.
The metal layers contain, for example, of tungsten. Alternatively, the metal layers may be formed of molybdenum or platinum.
The insulating layers contain, for example, of SiO2 or of silicon nitride.
The auxiliary layers contain, for example, of SiO2 or of silicon nitride.
To ensure that the recesses are of a defined depth, the auxiliary layers may be selectively etchable with respect to the insulating layers. To produce the recesses, the auxiliary layers are etched selectively with respect to the insulating layers.
Alternatively, an etching stop layer that can be etched selectively with respect to the auxiliary layer is produced between the auxiliary layer and the insulating layer disposed below it. To produce the recess, the auxiliary layer is etched selectively with respect to the etching stop layer until a part of the etching stop layer has been uncovered.
The part of the etching stop layer can then be removed or acts as part of the insulating layer.
It is preferable for there to be two or three metal layers.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for producing an acoustic mirror. The method includes the steps of providing a substrate, providing a first insulating layer on the substrate, providing a first auxiliary layer on the first insulating layer, forming a first recess in the first auxiliary layer, depositing a first metal layer over the first auxiliary layer and the first recess, abrading by chemical mechanical polishing the first metal layer until parts of the first metal layer disposed outside the first recess have been removed, and providing a second insulating layer over the first metal layer in the first recess and the first auxiliary layer. The second insulating layer has a thickness substantially equivalent to a thickness of the first insulating layer. A second auxiliary layer is provided on the second insulating layer. A second recess is formed in the second auxiliary layer. A second metal layer is deposited over the second insulting layer and in the second recess. The second metal layer has a thickness substantially equivalent to a thickness of the first metal layer. The thickness of the first and second metal layers and the first and second insulating layers are such that a reflection condition is satisfied. The second metal layer is abraded by chemical mechanical polishing until parts of the second metal layer disposed outside the second recess have been removed.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in an acoustic mirror and a method for its production, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.